I'll be totally honest and say I have no clue. I can feel it at about 35mph and it does it particularly bad at 45-55mph. It feels like driving on a kinda rough road, or maybe driving a car with flat spots in the tires, but the ones on the front are new and the rear ones have no obvious dips or abnormal wear. I can feel it in the seat and the steering wheel shakes, but I have never found any play in the front end.

 

To check your u-joints get under your truck on your back (if you can) and grab a good hold on your drive shaft. Then try and move your drive shaft back and forth, or side to side, up and down, and try to spin it. There may be some play when you try to rotate it, but the play should be in the differential housing. The play your looking for will be at the joint itself. I made a quick image of where your going to look for play. It could be at the transmission side or it could be at the differential side or both. If there is ANY movement at the u-joints then they are bad and need to be replaced.
(I know the image has a transmission but use your imagination)


Does that make sense?

 

Might sound expensive, but I know how a guy gets when he's not happy about something with his vehicle. You might try going back to the tire store and see if they'll spin each tire while it's mounted on your truck and see if one of them is out of round. The store where you bought the tires might do it for you for free if they have a balancer that balances the tires while they're on your truck. I don't know if they even still do that, but they used to. A tire can be balanced dead nuts even if it's out of round, and if it's out of round it will contact the road un-evenly and make it shake like you described - even though it's perfectly balanced. You will be able to see it, tell them you want to watch when or if they can do it.

 

That kinda goes back to the old saying "you can balance a square but it won't roll right..."

 

I did just have a thought....I replaced the pinion seal not too long ago. Its possible I didn't install the driveshaft the way I took it off. I don't think that it would cause an issue but I suppose it might.

 

^^ If the drive shaft isn't properly centered, It will cause a vibration.

 

Centered yes. I meant how you can install the driveshaft in one particular orientation and also 180 degrees too.

 

I wish I had a good ole T-5 in my truck instead of the 46 haha

 

I meant on the U-Joints, If one side is farther done than the other, that will cause a vibration. Is the centering straps aren't positioned right, or if one is missing, that will cause a vibration.
Check the torque on the u-joint straps/bolts. They may have become loose.

 

I'll look at it tomorrow and see what the deal is. If I remember right most Dodge/Chrysler vehicles use a big flage looking thing to bolt to a round plate on the pinion. Its been awhile since I've looked at one tho.